Index: cmake/Modules/CompilerRTDarwinUtils.cmake
===================================================================
--- cmake/Modules/CompilerRTDarwinUtils.cmake
+++ cmake/Modules/CompilerRTDarwinUtils.cmake
@@ -263,6 +263,7 @@
   set(CMAKE_C_FLAGS "-fPIC -fvisibility=hidden -DVISIBILITY_HIDDEN -Wall -fomit-frame-pointer")
   set(CMAKE_CXX_FLAGS ${CMAKE_C_FLAGS})
   set(CMAKE_ASM_FLAGS ${CMAKE_C_FLAGS})
+  append_string_if(COMPILER_RT_HAS_STD_C99_FLAG -std=c99 maybe_stdc99)
 
   set(PROFILE_SOURCES ../profile/InstrProfiling 
                       ../profile/InstrProfilingBuffer
@@ -283,7 +284,7 @@
                               OS ${os}
                               ARCH ${arch}
                               SOURCES ${filtered_sources}
-                              CFLAGS -arch ${arch}
+                              CFLAGS -arch ${arch} ${maybe_stdc99}
                               PARENT_TARGET builtins)
     endforeach()